Porous carbon-carbon composites may be used as electrode substrates, which are also known as gas diffusion layers (GDL's), in a variety of electrochemical cells such as proton exchange membrane fuel cells, phosphoric acid fuel cells, direct methanol fuel cells, as the cathode in cells used to electrolyze water and as electronic conductors in a variety of batteries. The carbon-carbon composites are typically formed from a mixture of carbon fibers and a thermosetting resin. Some substrates may contain carbon or graphite powders in addition to the carbon fibers and thermosetting resins. A good example of the structure of a fuel cell substrate, and a process used to form these substrates, are shown in U.S. Pat. No. 4,851,304. A typical method of making a substrate consists of: (1) forming a non-woven felt from a chopped carbon fiber and a temporary binder by a wet-lay paper making process in an aqueous suspension, (2) impregnating the felt with a phenolic resin dissolved in a solvent, followed by solvent removal without curing the resin so as to form the prepreg, (3) pressing one or more layers of prepreged felt to obtain both a desired thickness and a desired porosity, at a temperature sufficient to cure the resin, (4) heat treating in an inert atmosphere to between 750-1000° C. to convert the phenolic resin to carbon and (5) heat treating in an inert atmosphere to between 2000-3000° C., which partially graphitizes the carbon, to improve thermal and electrical conductivities and to improve corrosion resistance.
The art as illustrated by the aforementioned patent is technically acceptable, but is unacceptably expensive. About sixteen percent of the cost of the aforementioned process is for impregnating the carbon fiber paper with phenolic resin. Attempts to add powdered phenolic resin into the paper-making process, have not been successful. This is partly due to the high resin content resulting in sticking of the felt to the rolls on the paper-making machine. The powdered resins migrate to the surface of the felt during the drying portion of the paper-making process. This results in a high surface concentration of resin which causes sticking. It is also partly due to the impossibility of removing all of the phenolic powder from the aqueous suspension, which leads to environmental problems.